I do not believe that this is the reason. I have an Huawei Ascend Y100 (no front camera) that Skype works perfectly on. Cost me AUD$29.
I think the problem with the 8520 is it is just an unsupported model.
For those of you who have not seen what I am adding to all threads related to BlackBerries running OS 6 or 7 ...
One of my correspondents reminded me there was a time we were anticipating being able to use Skype on our older BlackBerries. Skype has not made any recent announcements about plans to release versions of the Skype app for older BlackBerry OS devices - that includes OS 6 and 7. Skype Lite, upon which we were able to use Skype, was decommissioned in January 2013: https://support.skype.com/en/category/SKYPE_LITE/
Skype is now available on the BlackBerry Z10 and Q10 running at minimum the 10.1 OS.